Revert "Auto-adjust fecdec output buffer size."

Revert "Auto-adjust fecdec output buffer size."

This reverts commit 35f9051506345255c7ed3c076b7df5f3b7d26039.

Currently, filters must not change their output buffer on the fly
because the writer might already have a reference to the old buffer
and keeps using this buffer.

As dealing with changing output buffers requires much more work,
including changes to the generic filter and writer code, let's defer
this change to post 0.3.4.